1. Technical Field
The present invention relates to an endoscopic device and an operating method thereof.
2. Related Art
An electronic endoscopic device includes an image capturing unit mounted at a tip portion of an endoscopic scope which is inserted in a body cavity of a subject. An image signal which is captured by the image capturing unit is transmitted to a processor device to which the endoscopic scope is connected, processed and then displayed on a display unit.
The image capturing unit which is used for the endoscopic device is mainly divided into a CCD type and a CMOS type but the CMOS type has been mainly used in recent years. A driving method which reads out the image captured signal to the outside in the CMOS type image capturing unit includes a global shutter method and a rolling shutter method.
The global shutter method concurrently resets all pixels in one screen to start an exposure operation in the CMOS type image capturing unit, that is, starts the accumulation of charges for all pixels in one screen. Therefore, the global shutter method has an advantage in that timings and periods of all pixels (photodiode: PD) from the start of the accumulation of the image capturing unit to the end of the accumulation of the image capturing unit become equal to each other. However, it is required to provide a primary capacity and an FET which transmits the primary capacity in an area of each of the pixels, which may affect the large area of the image capturing unit. Therefore, the configuration may be disadvantageous to mount the image capturing unit in an endoscopic scope with a small diameter.
According to the rolling shutter method, a CMOS type image capturing unit sequentially performs the exposure on at least one scanning line or each pixel so that the scanning lines or the pixels are sequentially reset and the charges start to be accumulated to read out the accumulated charges (also referred to as a focal plane shutter method).
The rolling shutter method is generally used for a digital camera but when the rolling shutter method is applied to the endoscopic device, a problem may occur. The endoscopic scope is inserted in a dark portion such as an inside of the body cavity so that illumination light needs to be irradiated from the tip portion of the scope to capture an image. Pulse light which is flickering light may be used as the illumination light as described in Patent Literature 1 (JP-A-2011-30985). If an on/off timing of the pulse light is not synchronized with a sequential read-out timing of the rolling shutter, a problem may be caused by an amount of illumination light.
FIG. 7 is a diagram illustrating an example of a problem of the rolling shutter method. In the case of the rolling shutter method, even though timings of starting the accumulation of the charges may be deviated in the order of the scanning lines (horizontal pixel row), it is required to irradiate the same amount of light for every scanning line.
The illumination light is pulse modulation driven pulse light. Therefore, pulse widths, the number of pulses, and pulse densities of the driving signals may be controlled to be equal to each other so that total light amounts of the respective scanning lines become the same during the exposure period. However, an amount of irradiated pulse light needs to be dynamically changed in accordance with a distance to a subject or luminance of an image to be observed so that it is difficult to control to uniformly assign the pulse light in the respective scanning lines whose read-out orders are shifted a little bit. If the amount of irradiated pulse light is not controlled to be the same in the respective scanning lines while controlling the irradiation amount to be changed, a luminance irregularity or a stripe pattern is generated in the image to be observed so that a quality of the captured image deteriorates.
FIG. 8 is a diagram illustrating another problem of the rolling shutter method. FIG. 8 illustrates an endoscopic device which alternately uses a plurality of light sources which has different emission colors to irradiate pulse light as an example. In the example illustrated in FIG. 8, an operation of irradiating for 1/60 second by a light source A twice and an operation of irradiating for 1/60 second by a light source B twice are alternately repeated. In the rolling shutter type image capturing unit, a timing of starting the accumulation of the charges and a timing of ending the accumulation of the charges are shifted in the order of the scanning lines so that an invalid frame image in which the pulse light irradiation by the light source A and the pulse light irradiation by the light source B are mixed is generated for every frame. The invalid frame image (denoted by “NG” in the drawing) is not used as an observation image of the endoscope so that a valid frame rate which is a ratio of valid frame images (denoted by “OK” in the drawing) is lowered to be half.
Such a problem may be solved by adopting a driving method of FIG. 9. For example, a frame period of 1/60 second is divided into a front period of 1/120 second and a back period of 1/120 second and the accumulation of the charges in each scanning line starts at the same time and the pulse light is irradiated for the first period of 1/120 second. Then, the accumulated charges of each scanning line may be read out in the order of the scanning lines during the next back period of 1/120 second.
The driving method illustrated in FIG. 9 may be performed in an image capturing unit with approximately 0.3 million recorded pixels which is currently widely used in the endoscopic device without any problems. However, it is difficult to apply the driving method to a high pixel image capturing unit offering one million pixels or more which may capture an image with a high definition (HD) image quality. This is because in the case of the high pixel image capturing unit offering one million pixels or more, an amount of data which is read out from all pixels is huge so that it is difficult to output all signals of the accumulated charges from the image capturing unit and transmit the signals to a processor device during the back period of 1/120 of the frame period. Hereinafter, an example that the number of recorded pixels is 1.3 million pixels will be described.
There is a limitation in increasing a transmission rate in order to transmit the image captured signal to a main body of the endoscopic device which is 3 to 4 m apart from the tip portion of the endoscopic scope so that a time to transmit a signal is elongated. Even though the problem of the time to transmit the signal may be solved by increasing the number of signal lines for transmitting the signal to perform parallel transmission, if the number of signal lines is increased, a diameter of an insertion unit of the endoscopic scope in which the signal lines are accommodated is increased. Therefore, the addition of the signal lines may be against the demand for the reduction of the diameter of the insertion unit.